Measurement of the $^{235}$U(n, f) cross section relative to the $^{6}$Li(n, t) and $^{10}$B(n, α) standards from thermal to 170 keV neutron energy range at n_TOF

The$^{235}$U(n, f ) cross section was measured at n_TOF relative to $^{6}$Li(n, t) and $^{10}$B(n,α) , with high resolution ( L=183.49(2) m) and in a wide energy range (25meV-170keV) with 1.5% systematic uncertainty, making use of a stack of six samples and six silicon detectors placed in the neutron beam. This allowed us to make a direct comparison of the yields of the$^{235}$U(n, f ) and of the two reference reactions under the same experimental conditions, and taking into account the forward/backward emission asymmetry. A hint of an anomaly in the 10-30keV neutron energy range had been previously observed in other experiments, indicating a cross section systematically lower by several percent relative to major evaluations. The present results indicate that the cross section in the 9-18keV neutron energy range is indeed overestimated by almost 5% in the recently released evaluated data files ENDF/B-VIII.0 and JEFF3.3, as a consequence of a 7% overestimate in a single GMA node in the IAEA reference file. Furthermore, these new high-resolution data confirm the existence of resonance-like structures in the keV neutron energy region. The results here reported may lead to a reduction of the uncertainty in the 1-100keV neutron energy region. ... mehrFinally, from the present data, a value of 249.7±1.4(stat)±0.94(syst) b·eV has been extracted for the cross section integral between 7.8 and 11eV, confirming the value of 247.5±3 b·eV recently established as a standard.